Light-emitting devices may be generally divided into organic light-emitting devices in which a light-emitting layer is formed from an organic material, and inorganic light-emitting devices, in which a light-emitting layer is formed from an inorganic material. Organic light-emitting diodes (OLEDs), a component of organic light-emitting devices, are self-emitting light sources based on the radiative decay of excitons in an organic light-emitting layer, the excitons being generated by the recombination of electrons injected through an electron injection electrode (cathode) and holes injected through a hole injection electrode (anode). OLEDs have a range of desirable qualities, such as low-voltage driving, rapid response times, ability to self-emit light while providing a screen having a wide viewing angle, high resolution, and natural color reproducibility.
Recently, research into applying OLEDs to a variety of devices, such as personal data assistances (PDAs), cameras, watches, office equipment, vehicle dashboard display devices, televisions (TVs), display devices, lighting systems, and the like has been actively undertaken.
In order to improve the luminous efficiency of OLEDs, it is necessary to improve the luminous efficiency of a material constituting a light-emitting layer or to improve light extraction efficiency in terms of a level at which light generated by the light-emitting layer is extracted.
Here, light extraction efficiency depends on the refractive indices of the layers of materials that constitute an OLED. In a typical OLED, when a beam of light generated by the light-emitting layer is emitted at an angle greater than a critical angle, the beam of light may be totally reflected at the interface between a higher-refractivity layer, such as a transparent electrode layer, and a lower-refractivity layer, such as a glass substrate. This consequently lowers light extraction efficiency, thereby lowering the overall luminous efficiency of the OLED, which is problematic.
More specifically, only about 20% of light generated by an OLED is emitted outwardly while about 80% of the light generated by the OLED is lost due to a waveguide effect originating from the difference in refractive indices between a glass substrate and an organic light-emitting layer that includes an anode, a hole injection layer (HIL), a hole transporting layer (HTL), an emission layer (EML), an electron transporting layer (ETL), and an electron injection layer (EIL), as well as by the total internal reflection originating from the difference in refractive indices between the glass substrate and the ambient air. Here, the refractive index of the internal organic light-emitting layer ranges from 1.7 to 1.8, whereas the refractive index of indium tin oxide (ITO), generally used for the anode, is about 1.9. Since the two layers have a significantly low thickness, ranging from 200 nm to 400 nm, and the refractive index of the glass used for the glass substrate is about 1.5, a planar waveguide is thereby formed inside the OLED. It is estimated that the ratio of the light lost in the internal waveguide mode due to the above-described reason is about 45%. In addition, since the refractive index of the glass substrate is about 1.5 and the refractive index of the ambient air is 1.0, when light exits the interior of the glass substrate, a beam of light having an angle of incidence greater than a critical angle is totally reflected and trapped inside the glass substrate. The ratio of the trapped light is commonly about 35%, and only about 20% of generated light is emitted outwardly.
Recently, the worldwide trend in lighting systems and display systems employing such OLEDs is toward large areas. However, when lighting or display systems are fabricated to have large areas, risks are increased due to the sagging of substrates, increases in equipment prices, and so on. It is also difficult to fabricate large-area lighting or display systems. Therefore, in reality, there are limitations to large-area lighting or display systems.